<!DOCTYPE html>
<html>
<style>
#flex {
  display: flex;
  width: 100px;
}
#flex > div {
  height: 30px;
}
.column {
  flex-direction: column;
}
</style>
<body>
<div id="flex">
    <div>Foo</div>
    <div id="target" data-offset-y=38>Bar</div>
</div>
<script src="../../resources/check-layout.js"></script>
<script>
var flexbox = document.getElementById('flex');
flexbox.offsetTop;
flexbox.classList.add('column');
checkLayout('#target');
</script>